1. Field of the Invention
The present invention relates generally to a method and apparatus for regulating the speed of a conveyor, and more particularly to an improved method and apparatus for regulating the speed of a screw conveyor arranged to supply plasticized material to a variable-volume chamber in extrusion apparatus, particularly that used in blow-molding operations.
2. Description of the Prior Art
In blow-molding operations, a cylindrical preform or parison is initially extruded, and then inflated to the desired shape in a blow-mold. In such extrusion apparatus, a screw conveyor is arranged to convey plasticized material to a variable-volume chamber formed by a piston slidably mounted in a cylinder. Between discharge operations, the conveyor supplies material to fill the chamber. The piston may be moved in the opposite direction to discharge material from the chamber. In order to use the apparatus as efficiently as possible, material should be discharged into the mold as soon as the mold is ready to receive such material. Hence, the speed of the conveyor should be set to convey a desired volume of material to the chamber in the available fill time. Any deviation in such conveyed volume of material may adversely affect performance of the apparatus.
In one known form of such apparatus, the degree of chamber filling is sampled at the beginning of each discharge operation. In other words, conveyor speed is determined by plunger position at the beginning of the discharge operation. If any deviation from a desired degree of chamber filling is detected, the speed of the conveyor is appropriately varied to increase or decrease the volume of material thereafter supplied to the chamber. However, it has been found that this known type of regulator is unstable, constantly over corrects, and causes the plunger stroke to fluctuate about a mean value. This causes undesirable vibration and oscillation, particularly if the parison volume is relatively large.
In another known type of such apparatus, the conveyor speed is not regulated, but held constant. The material supplied by the conveyor pushes the plunger back to a specific location, indicating availability for discharge. However, if the conveyor speed is set too slow, the mold may be in a "ready" condition before the chamber is filled, and a waiting interval must be tolerated. Conversely, if the conveyor speed is set too high, the chamber may be filled excessively and too much material may be discharged if the plunger is pushed forwardly to a specific location or mark on the cylinder.